1. Field of the Invention
The present invention relates to a navigation device which can be mounted on a vehicle such as a motor vehicle, for displaying the current position of the vehicle and traffic information received by the vehicle on a map displayed on the screen of a display device.
2. Description of the Prior Art
Referring now to FIG. 26, it illustrates a block diagram showing the structure of a prior art navigation device disclosed in, for example, Japanese Patent Application Laying Open (KOKAI) No. 4-40596. In the figure, reference numeral 1 denotes a display means such as a cathode ray tube display or a liquid crystal display, for displaying display data such as a map and the current position of a vehicle comprising such the prior art navigation device, 2 denotes a map storing means such as a compact disc or a semiconductor memory, for storing data on a map to be displayed on the screen of the display means 1, 3 denotes a selection switch for inputting various instructions provided by a driver, and 4 denotes a scale setting means for setting the scale of a map to be displayed on the screen of the display means 1 in accordance with an user's operation of the selection switch 3.
Furthermore, reference numeral 5 denotes a current position estimating means for estimating the current position of the vehicle. The current position estimating means is adapted to estimate the current position of the vehicle with high accuracy by using self-contained dead reckoning navigation with which the current position of the vehicle can be estimated on the basis of map data stored in the map storing means 2 and measured data by various sensors mounted on the vehicle, such as a magnetic sensor and a wheel speed sensor, and satellite navigation with which the current position of the vehicle can be estimated on the basis of data included in received radio waves delivered by satellites, in combination. Reference numeral 6 denotes a map reading means for reading needed map data from the map storing means 2 in response to output signals from the scale setting means 4 and current position estimating means 5, and 7 denotes an image converting means for converting map data read by the map reading means 6 into image information to be output to the display means 1.
Reference numeral 8 denotes a destination inputting means through which a driver inputs a destination, 9 denotes a distance determining means for determining the distance from the current position of the vehicle to the destination on the basis of position data output by the current position estimating means 5 and destination data input through the destination inputting means 8, 10 denotes an information storing means for storing road traffic information such as road information input in advance and traffic information input sequentially as needed, 11 denotes an information reading means for reading data which are associated with the current position from the information storing means 10, and 12 denotes an information restricting means for judging the importance of the road traffic information on the basis of data about the information output by the information reading means 11 and data about the distance output by the distance determining means 9 so as to prevent some pieces of road traffic information of little importance from being output to the display means 1 through the image converting means 7.
Next, a description will be made as to the operation of the prior art navigation device. FIGS. 27 through 30 are views of examples of a display of road traffic information added to a map displayed on the screen of the display means 1 of the navigation device shown in FIG. 26. Reference characters A to F denote respective pieces of road traffic information displayed on the screen of the display means 1, and A shows the current position of the vehicle, B shows a piece of information about a traffic jam, C shows a piece of information about a traffic regulation or control, D shows a specified area surrounded by dashed lines each of which forms a predetermined angle with the moving direction of the vehicle, E shows a recommended route to the destination, and F shows a route along which the vehicle is scheduled to move.
First, when the map reading means 6 receives a setting of the scale of a map set by a user through the scale setting means 4 and the current position of the vehicle estimated by the current position estimating means 5, it reads the map data on a map to be displayed on the screen of the display means 1 from the map storing means 2 in accordance with the scale of the map and current position of the vehicle. Then, the device displays the map on the screen of the display means 1 on the basis of the map data read out of the map storing means, and displays road traffic information on the map on-screen. Displaying all pieces of road traffic information increases the complexity of the screen display and hence causes a reduction in the visibility. To eliminate this problem, the information restricting means 12 prevents some pieces of road traffic information of little importance from being displayed on the display means 1.
That is, as shown in the example of FIG. 27, the device is adapted to set a specified area D surrounded by dashed lines each of which forms a predetermined angle with the moving direction of the vehicle and display only some pieces of road traffic information (i.e., three symbols B each showing a piece of traffic-jam information and two symbols C each showing a piece of traffic control information) which exist within the area D rather than all pieces of road traffic information. Furthermore, in the example shown in FIG. 28, the device is adapted to determine whether or not it displays each of all symbols B indicating a traffic-jam in accordance with the length of each of the traffic-jams. The device shows only traffic-jam symbols B each showing a traffic jam with a length which is equal to or longer than a predetermined length on the display means 1 so as to restrict the display of other traffic-jam symbols B each showing a traffic jam with a length which is shorter than the predetermined length.
Alternatively, as shown in FIGS. 29 and 30, when either a recommended route E from the current position A to the destination or a route F along which the vehicle is scheduled to move is displayed on the screen, the device judges the importance of road traffic information with reference to the recommended route E or route F along which the vehicle is scheduled to move so as to conclude that some road traffic information symbols other than other road traffic information symbols which exist in the vicinity of the recommended route E or on the route F along which the vehicle is scheduled to move are of little importance. Thus, the device restricts the display of the road traffic information symbols of little importance to display only the other road traffic information symbols, which exist in the vicinity of the recommended route E or on the route F along which the vehicle is scheduled to move, on the map.
Since such a prior art navigation device is so constructed as mentioned above, there is a problem in that though the device improves the visibility of the display screen by judging the importance of road traffic information and then restricting the output of some pieces of road traffic information which are concluded to be of little importance to the display means 1, the device cannot provide rich amounts of information.